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@Scott_Draper  Sounds like either the back pack is not adjusted properly, doesn't fit or is not up to the task of carrying that weight.  Osprey rate the Osprey Stratos 24 to 25lb I think.

which probably means it will not be that comfortable with 40lb.  From the looks of it the hip belt is not that beefy enough and I doubt the framing is stiff enough for that weight.   It may be ok for short lengths of time say < 1hour but ymmv.

In any case it won't be comfortable for long if it is not the right size or is adjusted incorrectly.

The most important measurement on sizing is the torso length.  If this is wrong then the backpack will not sit right on the hips.  It will either tend to carry on the shoulders if too short or slip down if too long.  This can make the shoulder straps uncomfortable because they won't adjust in their intended range.  The Stratos is a one size bag which means it fits you or it doesn't.  That is fairly typical of day packs that are not designed to carry much weight because up to about 20lbs poor fit is less of a problem.  The average male torso length seems to be around 19 inches so my guess is this pack is designed around that but it is just a guess. Osprey does not give a size range.

To put a backpack on,  you start with the shoulder straps and load lifters loose and tighten the hip belt so that the the backpack's weight is supported on your hips.  The belt should wrap around the top of your hip bones.  Then you snug up the shoulder straps lightly and loosely attach the sternum strap to just keep the shoulder straps from separating.  Then tighten the load lifters until they are just snug.  Make minor adjustment as required.  You should not feel much weight directly on your shoulders.  If you do, then try again starting from the beginning.  If you cannot get it to be comfortable then possibly the packs fit is wrong

You might be able to improve things by packing the weight differently.  You are concentrating the weight which is not really the type of load these backpacks are designed for.  As far as you can you want put the weight as close to your lower and middle back as possible.  To fill the pack you may need to pack the weight with blankets or similar to suspend it in the the right place then use the pack's compression straps to fix everything in place.   It may take a few tries to figure out the best way to do this so the weight doesn't slip down.